Digital cameras have become remarkably popular in recent years. A digital camera converts an optical image into an electrical signal using a picture image element such as a CCD (charge-coupled device) and records such as image data. In addition to miniaturization of the picture image element and higher pixel conversion, compactness of the digital camera is demanded. As a result of this demand, there is a need for compactness of the photographic lens that is mounted into a digital camera.
A rear-focus-type zoom lens with four lens groups for use in a video camera is known. For example, a known video camera has its entire optical system formed of four lens groups, with the first lens group and the third lens group being fixed. Zooming is performed by moving the second lens group along the optical axis along with movement of the fourth lens group, and focusing is performed by moving the fourth lens group. This particular type of zoom lens has a large zoom ratio, and is disclosed, for example, in Japanese Laid Open Patent Application H11-194269.
However, conventional zoom lenses tend to be too long, and particularly in compact digital cameras, excessive length in order to obtain good optical performance is unacceptable. For example, with the zoom lens described in Japanese Laid Open Patent Application H11-194269, since the most object-side surface of the second lens group is convex, the focal length of the second lens group becomes longer, and the movement of the second lens group at the time of zooming becomes large. Therefore, the entire length of the lens becomes long, contrary to the compactness desired for an imaging lens in a digital camera.
Japanese Laid Open Patent Application 2001-215408, the inventor of which is the same as that of the present application, describes a compact zoom imaging lens for use in a digital camera with favorable optical performance having a zoom ratio of five to six. However, with that lens, the focal length of the fourth lens group is short, especially when attempting to obtain a high zoom ratio that exceeds six. Thus, large variations in aberrations occur due to focusing and correcting the image point position during zooming. Therefore, there is market demand for a zoom lens of small overall length that can achieve a high zoom ratio (i.e., in the range six to eight) with only small fluctuations in aberrations.
The present invention is a four-group zoom lens of the rear-focus-type that is ideally suited for use in a digital still camera. More specifically, the present invention has a small overall length and a zoom ratio in the range of six to eight. The zoom lens of the present invention can be mounted in a digital camera so as to provide reduced variations in aberrations during zooming and focusing and is of shorter overall length than previous zoom lenses of comparable optical performance.